This article will talk about one specific category of lithium-ion batteries; Lithium-Iron-Phosphate or LiFePO4 in its chemical formula, also abbreviated as LFP batteries. These are a little different from what you have in your cell phone and laptop, those are (mostly) lithium-cobalt batteries. The advantage of LFP is that it is much more stable, and not prone to self-combustion. That does not mean the battery cannot combust in case of damage: There is a whole lot of energy stored in a charged battery and in case of an unplanned discharge the results can get very interesting very quickly! LFP also lasts longer in comparison to lithium-cobalt, and is more temperature-stable. Of all the various lithium battery technologies out there this makes LFP best suited for deep-cycle applications!

Why Lithium-Ion?

It is too easy to pooch an expensive lead-acid battery bank in mere months by letting it sit at partial charge. That is very different for LFP! You can let lithium-ion batteries sit at partial charge forever without damage. In fact, LFP prefers to sit at partial charge rather than being completely full or empty, and for longevity it is better to cycle the battery or to let it sit at partial charge.

Lithium-ion batteries are very nearly the holy grail of batteries: With the right charge parameters you can almost forget there is a battery. There is no maintenance. The BMS will take care of it, and you can happily cycle away!

LFP batteries can also last a very long time. Our LFP batteries are rated at 3000 cycles, at a full 100% charge/discharge cycle. If you did that every day it makes for over 8 years of cycling! They last even longer when used in less-than-100% cycles, in fact for simplicity you can use a linear relationship: 50% discharge cycles means twice the cycles, 33% discharge cycles and you can reasonably expect three times the cycles.

lithium ion golf cart battery

How Does a LiFePO4 Battery Work?

If you have really been paying attention you now understand that the battery drawing on the right shows an LFP battery that is almost completely discharged. Nearly all the lithium ions are on the side of the positive electrode. A fully charged battery would have those lithium ions all stored inside the carbon of the negative electrode.

In the real world lithium-ion cells are built of very thin layers of alternating aluminum – polymer – copper foils, with the chemicals pasted on them. Often they are rolled up like a jelly-roll, and put in a steel canister, much like an AA battery. The 12 Volt lithium-ion batteries you buy are made of many of those cells, connected in series & parallel to increase the Voltage and Amp-hour capacity. Each cell is around 3.3 Volt, so 4 of them in series makes 13.2 Volt. That is just the right Voltage for replacing a 12 Volt lead-acid battery!

We hinted at this above: Lithium-ion batteries have 100% usable capacity, while lead-acid really ends at 80%. That means you can size an LFP battery bank smaller than a lead-acid bank, and still have it be functionally the same. The numbers suggest that LFP can be 80% the Amp-hour size of lead-acid. There is more to this though.

For longevity lead-acid battery banks should not be sized where they regularly see discharging below 50% SOC. With LFP that is no problem! Round-trip energy efficiency for LFP is quite a bit better than lead-acid as well, meaning that less energy is needed to fill up the tank after a certain level of discharge. That results in faster recovery back to 100%, while we already had a smaller battery bank, reinforcing this effect even more.

The bottom line is that we would be comfortable to size a lithium-ion battery bank at 55% – 70% of the size of an equivalent lead-acid bank, and expect the same (or better!) performance. Including on those dark winter days when sun is in short supply.

To sum up, for long and happy LFP battery life, in order of importance, you should be mindful of the following:

  1. Keep the battery temperature under 45 Centigrade (under 30C if possible) – This is by far the most important!!
  2. Keep charge and discharge currents under 0.5C (0.2C preferred)
  3. Keep battery temperature above 0 Centigrade when discharging if possible – This, and everything below, is nowhere near as important as the first two
  4. Do not cycle below 10% – 15% SOC unless you really need to
  5. Do not float the battery at 100% SOC if possible
  6. Do not charge to 100% SOC if you do not need it